Rails core - Jun 2014 - Mystery conditional on alias_method in DateTime core extension

I'm working on a proposed refactoring of to_formatted_s in the Date, Time 
and DateTime core extenstions in ActiveSupport.

I'm wondering if anyone can explain what the conditional is for here...  
https://github.com/rails/rails/blob/master/activesupport/lib/active_support/core_ext/date_time/conversions.rb#L40

I've traced it's existence back to this commit from Jeremy in 2008 but
the
commit message is a bit Greek to me. 
https://github.com/rails/rails/commit/047db112991785d871e9a6ba165b93752b0fff06

There's nearly identical code in the Date and Time extensions, but only the 
DateTime extension includes this mystery condition.  Any insights?

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